Many types of laser can be configured to deliver a beam in a single longitudinal mode with maximum beam quality. Beam quality is usually defined by a term M2. The beam quality M2 is often defined by the ratio of a diameter-divergence product of the beam to the ideal, (single-mode) diffraction-limited (TEM00) beam diameter-divergence product. It can also be defined as the square of the ratio of the beam diameter to the diffraction-limited beam diameter. An ideal diffraction-limited beam would have a value of M2 equal to 1.0. In a pure single-mode beam, the intensity profile transverse to the beam is Gaussian. If a pure single-mode beam is focused by a diffraction-limited optical system, the focused beam will have a Gaussian intensity profile. On either side of the focal plane, within several Rayleigh ranges in front of and behind the focal plane, the transverse energy distribution will also be essentially Gaussian. The Rayleigh range is the distance a beam must travel in order for the beam diameter to increase to 1.414 times the beam diameter at the waist.
A Gaussian or near-Gaussian intensity profile in a focused laser-beam is useful for applications such as laser drilling of very small diameter holes, for example holes having a diameter of a few micrometers (μm) or less. There are other applications, however, such as laser welding, laser material processing, and laser tissue treatment for which a broader, flat-topped or quasi-flat-topped intensity profile would be preferable. In many of these applications, a focused beam from a laser in a fixed position must be mechanically scanned over a surface being treated or along a seam being welded.
The prior-art includes several different arrangements for providing flat-topped or quasi-flat-topped intensity profile. These arrangements include the use of optical elements such as highly aspheric lens elements, diffractive optical elements, and beam-homogenizing devices such as microlens arrays, alone or in combination. All of these optical elements are usually more complex and expensive to produce, in comparable quality, than simple, spherical optical elements. Microlens-array type beam homogenizers are not suitable for use in an optical system wherein a focused beam must be scanned as discussed above.
There is a need for reducing the complexity and cost of optical systems that focus a laser-beam having a Gaussian or near-Gaussian intensity profile into an image plane such that the energy intensity profile of the beam in the image plane is quasi-flat-topped. Preferably, the complexity and cost reduction should be achieved while still providing a capability for scanning the laser-beam over a surface or tissue being irradiated by the beam.